In recent years, portable electrical devices are being greatly reduced in size and weight, and their power consumption is also increasing with increase in their functionality. Therefore, there is an increasing need for the nonaqueous electrolyte secondary batteries used as power sources to be reduced in weight and increased in capacity.
To increase the energy density of the nonaqueous electrolyte secondary batteries, it is necessary that one or both of the positive and negative electrode materials used have high energy density. In PTL 1 below, a plurality of oxides containing lithium and having the antifluorite structure are studied as positive electrode active materials.
PTL 1 proposes that an oxide containing at least two cationic elements in addition to lithium is used as the oxide having the antifluorite structure to thereby stabilize the crystal structure of the positive electrode active material. However, in PTL 1, since the oxide having the antifluorite structure is used as the positive electrode active material, it is assumed that the positive electrode active material is used at a positive electrode potential of less than 4.0 V (vs. Li/Li+) at which the oxide having the antifluorite structure is not decomposed. At such a positive electrode potential, the oxide having the antifluorite structure is not decomposed, and therefore the stability of decomposition residues is unknown. Since the oxide having the antifluorite structure is strongly basic, a problem may arise in that the oxide causes generation of gas unless the oxide is sufficiently reacted and decomposed.
Metals such as Si and Sn that are alloyed with lithium are studied as negative electrode materials. For example, a Si-based material has a theoretical capacity higher by a factor of 10 or more than those of existing graphite negative electrode materials. However, a problem with such a Si-based material is that, when it is used as a negative electrode material, its irreversible capacity, which is the amount of non-dischargeable capacity after a charge-discharge cycle, may be large, so that the above advantage cannot be utilized.
PTL 2 below proposes a technique for improving the irreversible capacity of such a Si-based negative electrode material. Specifically, an oxide having the antifluorite structure is mixed into the positive electrode active material to obtain a sufficiently high charge capacity to thereby compensate for the irreversible capacity of the negative electrode material, so that a high-energy density battery can be provided.
One known example of the oxide having the antifluorite structure is Li5FeO4. In a battery using Li5FeO4 as an additive for the positive electrode, when lithium is extracted from the Li5FeO4 during charging, not only the valence of the transition metal in its structure changes, but also the structure is decomposed and oxygen is released. In this case, lithium can be further extracted, so that the amount of lithium usable for charging and discharging in the battery is large. Accordingly, a high charge capacity can be obtained.